The present invention pertains to improvements in the field of wastewater treatment. More particularly, the invention relates to a biofiltering system for treating wastewater effluent leaving a septic tank.
Small wastewater treatment systems are typically designed so that raw wastewater generated by a residence is discharged directly into a septic tank. Once discharged into the septic tank, the effluent is allowed to partially settle and is then passed into a dosing chamber. From the dosing chamber, the effluent is fed into a filtering medium, e.g. a trickling bed filter, and then collected in a central drain pipe. Once the effluent is collected in the central drain pipe, it is then dispersed to a plurality of parallel drain pipes positioned generally equidistant from one another across a specified area underneath the ground. Each drain pipe has a plurality of orifices which allow the effluent to be released into the surrounding environment.
Various filtering media and devices have been proposed for treating the wastewater discharged from a septic tank. For example, U.S. Pat. No. 5,206,206 discloses the use of pre-treated peat in biofilters for wastewater treatment. The pre-treated peat comprises a mixture of pre-sieved peat with a Fe-containing compound and lime. A filter bed constituted of a layer of such a pre-treated peat disposed between upper and lower layers of calcareous stones is arranged inside a filtration column. A rotary distribution system including a perforated arm is used for distributing the wastewater to be treated on top of the filter bed. Since the pressure of the incoming wastewater serves to displace the perforated arm and the wastewater entering the distribution system contains suspended particles of organic and inorganic materials, the suspended particles often clog the rotation mechanism, thus rendering the distribution system inoperative.
U.S. Pat. No. 5,618,414 also discloses a wastewater treatment system utilizing peat as a filtering medium. Such a treatment system comprises a container having an upper portion, a lower portion, at least one waster water inlet in the upper portion of the container for receiving the wastewater, and an opening in the lower portion of the container for allowing the treated water to escape the container. At least one elongated hollow casing is mounted within the lower portion of the container. The casing has an open bottom surface and defines at least two treatment chambers within the container, each of the treatment chambers containing a bed of peat for treating the wastewater. At least one distribution means is also provided for distributing the wastewater entering the container through the wastewater inlet into at least one of the treatment chambers. Each distribution means comprises a water inlet pipe connected to the wastewater inlet and leading above the casing and a trough tiltably mounted on top of a corresponding casing. The trough extends along the casing and has two opposite sides. It defines at least one wastewater receiving means on one of the two sides and it is tiltable between a first position whereat the wastewater receiving means receives wastewater exiting the water inlet pipe and a second position whereat the wastewater received in the receiving means flows out of the same. The trough also has counterweight means on its other side for holding it in the first position while it is filled up and for bringing it back from the second position to the first position after the at least one wastewater receiving means has been emptied. In addition, at least one distribution plate is mounted above the bed of peat in one of the treatment chambers defined by the corresponding hollow casing. This at least one distribution plate comprises a plurality of channels projecting from the trough, each of the channels having an end for receiving wastewater flowing from the trough so that the wastewater is divided into a plurality of flows each flowing in a corresponding channel, each of the channels also having at least one opening for letting the wastewater drip into the corresponding treatment chamber.
The use of the aforesaid tiltable trough in combination with the distribution plate for distributing the wastewater over the bed of peat presents certain drawbacks. When the wastewater enters the container at a high flow rate and a large amount of wastewater is continuously received in the trough over a period of time, the trough rapidly tilts between the aforesaid first and second positions so that the wastewater flowing therefrom floods the channels defined in the distribution plate and flows over the edges of the distribution plate and onto the bed of peat. There is thus flooding of the bed of peat. Since any given peat has a predetermined biofiltering capacity over which the peat will not act as a biofiltering medium, flooding of the bed of peat will cause the wastewater to flow through the bed of peat at a flow rate greater than the biofiltering capacity of the peat, resulting in a non-efficient removal of both organic and inorganic materials.
In order to overcome the above drawbacks, Applicant has proposed in U.S. patent application Ser. No. 09/572,138 now U.S. Pat. No. 6,372,128 a biofiltering device comprising a housing having inlet means for receiving the wastewater to be treated and outlet means for discharging the treated wastewater, a bed of peat disposed inside the housing between the inlet and outlet means, the peat defining a biofiltering medium having a predetermined biofiltering capacity, and means for aerating the peat. Such a device further includes a fluid flow control system arranged over the bed of peat and in fluid flow communication with the inlet means, for distributing the wastewater through the bed of peat in a manner such that when the wastewater enters the system at a flow rate greater than the biofiltering capacity of the peat, the wastewater discharged from the system flows through the bed of peat at a flow rate no greater than the biofiltering capacity.
According to a preferred embodiment of the biofiltering device described in the aforesaid application, the fluid flow control system comprises a fluid flow control unit having a chamber of variable volume in fluid flow communication with the inlet means and a member provided with a plurality of spaced-apart discharge orifices in fluid flow communication with the chamber and facing the bed of peat, the volume of the chamber varying in response to a variation of the flow rate of the wastewater passing through the chamber and increasing when the flow rate of the wastewater is greater than the biofiltering capacity. The discharge orifices each have a dimension selected so that the wastewater discharged from the flow control unit through the orifices flows through the bed of peat at a flow rate no greater than the biofiltering capacity of the peat, when the flow rate of the wastewater passing through the chamber is greater than the biofiltering capacity.
The above biofiltering device is typically capable of treating up to about 550 liters of wastewater per day. As described in U.S. patent application No. 09/572,138, now U.S. Pat. No. 6,372,128 a plurality of such devices can be connected by means of conduits to a distributor box which in turn is in fluid flow communication with the septic tank, for handling larger quantities of wastewater. Generally, up to ten biofiltering devices can be so connected in order to treat up to about 5,500 liters of wastewater per day. When it desired to treat on a daily basis quantities larger than 6,000 liters, the installation of more than ten biofiltering devices becomes costly and complex.
It is therefore an object of the present invention to overcome the above drawback and to provide an improved biofiltering system which utilizes peat as biofiltering medium and which can efficiently treat large quantities of wastewater irrespective of the flow rate thereof.
In accordance with the present invention, there is provided a biofiltering system for treating wastewater effluent for dissipation through an effluent absorption area and into earth beneath the effluent absorption area. The biofiltering system of the invention comprises a bed of peat overlying the effluent absorption area, the peat defining a biofiltering medium having a predetermined biofiltering capacity, and fluid flow regulating means for allowing the wastewater effluent to be treated to flow at a flow rate greater than the biofiltering capacity of the peat, while not exceeding a predetermined threshold value. The system according to the invention further includes a predetermined number of spaced-apart, elongated distribution conduits arranged over the bed of peat and in fluid flow communication with the fluid flow regulating means, each distribution conduit having a plurality of discharge orifices spaced longitudinally therealong, for distributing the wastewater effluent through the bed of peat in a manner such that wastewater discharged from the distribution conduits through the orifices flows through the bed of peat at a flow rate no greater than the biofiltering capacity of the peat.
According to a preferred embodiment of the invention, the number of distribution conduits, the inner dimension of each distribution conduit and the dimension of each discharge orifice are selected in accordance with the predetermined threshold value so that the wastewater discharged from the distribution conduits through the orifices flows through the bed of peat at a flow rate no greater than the biofiltering capacity of the peat, and preferably equal thereto.
According to another preferred embodiment, the distribution conduits are arranged in spaced-apart parallel relation to one another and are connected downstream to at least one header conduit in fluid flow communication with the fluid flow regulating means, the distribution conduits extending laterally of the header conduit. Preferably, there are two header conduits arranged in spaced-apart parallel relation to one another, the header conduits being connected downstream to a supply conduit extending laterally thereof; the supply conduit is connected downstream to a transport conduit in fluid flow communication with the fluid flow regulating means.
In a particularly preferred embodiment, the distribution conduits, the header conduits, the supply conduit and the transport conduit each have a circular cross-section with a respective inner diameter. The inner diameter of each distribution conduit is smaller than the inner diameter of each header conduit and the inner diameters of the header conduits and supply conduit are smaller than the inner diameter of the transport conduit. Such an arrangement enables the wastewater discharged through the orifices to flow at a substantially uniform flow rate.
According to yet another preferred embodiment, the fluid flow regulating means comprise a reservoir having inlet means for receiving the wastewater effluent to be treated and outlet means coupled to a valve for discharging the wastewater effluent at a flow rate greater than the biofiltering capacity of the peat, while not exceeding a predetermined threshold value; the transport conduit is connected downstream to the valve. Preferably, the outlet means include an outlet conduit connected upstream to the valve. Thus, when it is desired to treat about 6,000 to about 30,000 liters of wastewater per day, the valve is operated so that the flow rate of the wastewater effluent discharged from the reservoir will not exceed a threshold value within a range of about 250 to about 1250 l/hr. For a threshold value within such a range, the number of distribution conduits generally ranges from about 6 to about 18, and the inner diameter of each distribution conduit generally ranges from about 25 to about 50 mm. The discharge orifices, on the other hand, are generally circular and each have a diameter ranging preferably from about 2 to about 8 mm. When the peat used is sphagnum peat having a biofiltering capacity of about 20 l/hr, the orifices each have a diameter of about 6 mm. When using sphagnum peat having a biofiltering capacity of about 22 l/hr, the orifices each have a diameter of about 5 mm. In the case of sphagnum peat having a biofiltering capacity of about 24 l/hr, the orifices each have a diameter of about 3 mm. Such a selection of parameters ensures that the wastewater discharged from the distribution conduits through the orifices will flow through the bed of peat at a flow rate no greater than the biofiltering capacity of the peat.
According to still another preferred embodiment, the biofiltering system further includes fluid flow control means arranged between the bed of peat and the distribution conduits for receiving the wastewater discharged therefrom and causing the wastewater to flow throughout substantially the entire bed of peat. Preferably, the fluid flow control means comprises an elongated, horizontally extending porous membrane having upper and lower surfaces with the lower surface contacting the peat. The membrane is capable of spreading the flow of the wastewater discharged from the distribution conduits as the wastewater flows through the membrane from the upper surface to the lower surface, and into the bed of peat.
In a particularly preferred embodiment, the membrane is a multilayered membrane comprising upper and lower layers formed of non-woven fibers and each having a predetermined density, and an intermediate layer disposed between the upper and lower layers and formed of the non-woven fibers, the intermediate layer having a density less than the predetermined density. The upper and lower layers each have a plurality of spaced-apart apertures extending therethrough.
According to a further preferred embodiment, the bed of peat, the distribution conduits, the header conduits, the supply conduit, the transport conduit and the valve are disposed inside a bottomless ventilated housing resting on the effluent absorption area, the reservoir being disposed outside of the housing with the outlet conduit extending through an opening formed in a sidewall of the housing. In such an embodiment, the biofiltering system further includes means for aerating the peat. Preferably, the means for aerating the peat comprise a network of interconnected, open-ended aeration conduits each having first and second end portions and an elongated intermediate portion extending therebetween and horizontally through the bed of peat at a predetermined depth, the first and second end portions of each aeration conduit extending upwardly from the intermediate portion and having respective free ends disposed at a predetermined heights above the bed of peat with the free end of the first end portion being disposed above the free end of the second end portion so as to permit air circulation in the aeration conduits. The intermediate portion of each aeration conduit is provided with a plurality of aeration orifices spaced therealong for discharging air into the bed of peat.
According to still a further preferred embodiment, the biofiltering system further includes sampling means enabling a sample of treated wastewater to be collected for analysis. Preferably, the sampling means comprise a horizontal tray-like member disposed inside the housing adjacent an other sidewall thereof and at a predetermined depth in the bed of peat, a guide member connected to the tray-like member and a drip conduit in fluid flow communication with the guide member and extending through the other sidewall, the drip conduit having a free end disposed exteriorly of the housing. The tray-like member has a main fluid-receiving surface extending along an inclined plane for causing drops of treated wastewater received on the main surface to flow in a direction towards the other sidewall and the guide member has a guide channel arranged to receive the drops of treated wastewater from the main surface for guiding the drops into the drip conduit. Preferably, the tray-like member has two secondary fluid-receiving surfaces disposed opposite one another and each extending along an inclined plane for causing drops of treated wastewater received on the secondary surfaces to flow in a direction towards the main surface.
The biofiltering system according to the invention enables one to efficiently treat large quantities of incoming wastewater effluent irrespective of the flow rate thereof.